DocXtender does not add the ability to attach documents, it just adds a drag & drop interface. Attachments must be working in base Business Central before trying to use DocXtender.
Can the user manually attach documents to the page using the base Business Central drag & drop function? Click the “Attach file” action (or the + sign if links) to try both browsing and drag & drop to upload a file instead of using DocXtender. If not, are you trying to attach the file before the record has been created? If basic attachments are not working in base Business Central, you will need to contact Microsoft directly.
There was a known Microsoft issues in BC22 that may require clearing the browser cache and cookies before the attachment factbox can be personalized onto the page. Otherwise you would need to follow up with Microsoft to get the base attachments working first.